Why are Paul's writings not his own personal opinion or bragging?
Paul's writings are not his own personal opinion or bragging because every word he writes is Holy Spirit inspired, not from his own feelings. The Holy Spirit inspired him to write for our benefit, so the authority is divine rather than personal.
Les explains that Paul's writings would sound egotistical except for the fact that every word Paul writes is Holy Spirit inspired. It does not come from Paul's own feeling, and he is not bragging about what he is; rather, the Holy Spirit inspired him to write for our benefit. Les also points out that Paul constantly had to defend his apostleship because it was under Satanic attack, and that the Spirit inspired Paul to claim only that he, as a wise master builder, laid the foundation—not that he is the foundation.
For we are his workmanship, created in Christ Jesus unto good works, which God hath before ordained that we should walk in them.